areadlinkat: avoid failure on older glibc

areadlinkat: avoid failure on older glibc

Acting on /proc/self/-1/name gives ENOTDIR.  at-func normally falls
back to fchdir, which discovers the real problem of invalid fd and
changes to EBADF; but for areadlinkat, it mistakenly short-circuited
when FUNC_FAIL was NULL (since 0<=(char*)result is always true).

* lib/at-func.c (AT_FUNC_NAME): Check for explicit FUNC_FAIL,
rather than mis-comparing 0 against FUNC_RESULT of char*.

Signed-off-by: Eric Blake <ebb9@byu.net>